Bismuth-based chloride-storage electrodes

ABSTRACT

Bismuth-based, chloride-storage electrodes and rechargeable electrochemical cells incorporating the chloride-storage electrodes are provided. Also provided are methods for making the electrodes and methods for using the electrochemical cells to remove chloride ions from a sample. The chloride-storage electrodes, which are composed of bismuth metal, can store chloride ions in their bulk by forming BiOCl via an oxidation reaction with bismuth in the presence of an oxygen source.

REFERENCE TO GOVERNMENT RIGHTS

This invention was made with government support under DMR-1121288awarded by the National Science Foundation. The government has certainrights in the invention.

BACKGROUND

With the steady growth in human population, as well as rapid developmentof industry and agriculture, access to fresh water is a serious issue.Considering the abundance of seawater on Earth, desalination, whichseparates dissolved salt ions from a saline feed water source to producefresh water, is a viable option to achieve an adequate supply of freshwater. Reverse osmosis (RO) is considered an advanced technology for thedesalination of seawater. However, RO desalination requires considerableelectrical energy input. From the perspective of energy management,capacitive deionization (CDI), which removes salt ions by capturing themin the electrical double layer of high surface area electrodes, is ahighly promising alternative, as its operating cost can be reducedsignificantly compared to the operating cost of RO. However, CDI haslimitations in desalinating highly concentrated feed water, such asseawater, because the amount of salt ions that can be stored in theelectrical double layer is limited even when high surface areananoporous carbon electrodes are used. Therefore, CDI has been developedmainly for use in desalination of brackish water.

In order to increase the capacity of an electrode for electrochemicalsalt removal, salt ions should be stored not just in the double layerbut in the bulk of the electrode through the formation of chemicalbonds. For example, Pasta et al. constructed a desalination cell bycombining MnO₂ as the Na-storage electrode and Ag as a Cl-storageelectrode, where Na⁺ and Cl⁻ ions were stored within the electrodestructures forming new phases, Na₂Mn₅O₁₀ and AgCl, respectively. (M.Pasta, C. D. Wessells, Y. Cui, F. L. Mantia, A desalination battery,Nano Lett. 12, 839-843 (2012).) If the desalination cell can store andrelease salt ions repeatedly through the charging and dischargingprocess, and the discharging process can partially recover the energyconsumed during the charging process, such a cell can be considered arechargeable “desalination battery” and has the potential to achievedesalination with a minimum overall energy input. Considering that thehigh cost of Ag and the poor electrical conductivity of AgCl limit theuse of Ag for practical, large scale desalination cells, the developmentof desalination batteries depends on the discovery of more efficient,stable, and practical Cl-storage electrodes.

SUMMARY

Chloride-storage electrodes composed of bismuth metal andelectrochemical cells incorporating the chloride-storage electrodes,including rechargeable electrochemical desalination cells, are provided.Methods for making the electrodes and methods for using theelectrochemical cells to remove chloride ions from a sample, such asseawater, are also provided.

One embodiment of an electrochemical cell comprises: a chloride-storageelectrode comprising bismuth metal; an electrolyte solution comprisingchloride ions in contact with the chloride-storage electrode; a secondelectrode in electrical communication with the chloride-storageelectrode; and an electrolyte solution in contact with the secondelectrode. The electrolyte solution in contact with the second electrodecan be either the electrolyte solution comprising chloride ions that isin contact with the first electrode or a different electrolyte solution.

One embodiment of method for removing chloride ions from an electrolytesolution using an electrochemical cell of the type described hereincomprises generating a flow of electrons from the chloride-storageelectrode to the second electrode, whereby bismuth in thechloride-storage electrode is oxidized to form BiOCl and a reductionreaction occurs at the second electrode.

Other principal features and advantages of the invention will becomeapparent to those skilled in the art upon review of the followingdrawings, the detailed description, and the appended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative embodiments of the invention will hereafter be describedwith reference to the accompanying drawings.

FIG. 1A shows a Pourbaix diagram for Bi in pure water. FIG. 1B depicts aPourbaix diagram for Bi in a 0.6 M solution at 25° C.

FIG. 2A shows the morphology of a Bi electrode using a low magnificationscanning electron microscope (SEM) image showing the foam structure.FIG. 2B depicts the morphology of a Bi electrode using a highmagnification SEM image showing the nanocrystalline wall of the Bi foamelectrode. FIG. 2C shows a SEM image of a BiOCl electrode obtained by afirst Cl-storage process. FIG. 2D shows a SEM image of a Bi electrodeafter the first Cl-release process. FIG. 2E shows a SEM image of a BiOClelectrode after a second Cl-storage process. FIG. 2F shows a SEM imageof a Bi electrode after the second Cl-release process.

FIG. 3A depicts the electrochemical characterization of Linear SweepVoltammetry (LSV) of a Bi electrode to the positive direction and to thenegative direction (scan rate=5 mV s⁻¹). FIG. 3B shows potentialprofiles of a Bi electrode during galvanostatic oxidation and reductionat ±1 mA cm⁻² in 0.6 M NaCl. FIG. 3C depicts a cycle test of Bielectrodes using an initial capacity of 0.2216 mAh cm⁻² and 0.1756 mAcm⁻² for Cl storage.

FIG. 4A shows X-ray Diffraction (XRD) patterns for a Bi electrodeas-deposited (panel (a)); after the first chlorination (panel (b));after the first dechlorination (panel (c)); after the secondchlorination (panel (d)); and after the second dechlorination (panel(e)). FIG. 4B shows the capacity-potential plot recorded for eachprocess.

FIG. 5A shows potential-capacity plots of a Bi electrode for the firstthree chlorination/dechlorination cycles at ±2 mA cm⁻² in 0.6 M NaCl inthe voltage range of 0.5 V and −1.4 V vs. Ag/AgCl. FIG. 5B depictspotential-capacity plots of the Bi electrode for the first threechlorination/dechlorination cycles using the same conditions except thatthe initial chlorination capacity was restricted to 0.1756 mAh cm⁻²,which is 80% of the initial chlorination capacity shown in FIG. 5A.

FIG. 6A depicts the XRD pattern for NASICON-type NaTi₂(PO₄)₃. FIG. 6Bshows the potential-capacity plots for the first threesodiation/desodiation cycles for the NaTi₂(PO₄)₃. FIG. 6C depicts thecycle performance of the NaTi₂(PO₄)₃. The sodiation/desodiationreactions were performed at ±2 mA cm⁻² in 1 M Na₂SO₄ in the potentialrange of −1.0 to −0.4 V vs. Ag/AgCl.

FIG. 7A depicts the operation of a Type 1 desalination cell. FIG. 7Bshows the operation of a Type 2 desalination cell. FIG. 7C depicts theoperation of a Bi/NaTi₂(PO₄)₃ desalination cell, as described in theExample.

FIG. 8A depicts the performance of a Bi/NaTi₂(PO₄)₃ desalination cellwith potential-capacity plots at ±1 mA cm⁻² of Bi and NaTi₂(PO₄)₃measured vs. Ag/AgCl during the desalination process in 0.6 M NaCl (pH6.4). FIG. 8B shows the performance of the Bi/NaTi₂(PO₄)₃ desalinationcell with potential-capacity plots at ±1 mA cm⁻² of BiOCl andNa₃Ti₂(PO₄)₃ during the salination process in 0.6 M NaCl. FIG. 8Cdepicts the performance of the Bi/NaTi₂(PO₄)₃ desalination cell withpotential-capacity plots at ±1 mA cm⁻² of BiOCl in 70 mM HCl (pH 1.15)and Na₃Ti₂(PO₄)₃ in 1 M Na₂SO₄ during the salination process. FIG. 8Dshows cell voltage-capacity plots during the first threecharging-discharging cycles using the operating conditions shown in FIG.7C. FIG. 8E shows a cycle test of the Bi/NaTi₂(PO₄)₃ cell at ±1 mA cm⁻²using the operating conditions shown in FIG. 7C.

FIG. 9A shows LSV for the reduction of BiOCl in a 0.6 M NaCl solution(pH 6.4). FIG. 9B depicts the LSV for the reduction of BiOCl in a 70 mMHCl solution (pH 1.15). A scan rate of 5 mV s⁻¹ was used.

FIG. 10A shows the potential-capacity plots of a Bi electrode performingchlorination and a Pt electrode performing H₂ evolution in 70 mM HClduring charging. For comparison, FIG. 10B shows the potential-capacityplots of the Bi electrode performing chlorination in 70 mM HCl and theNaTi₂(PO₄)₃ electrode performing sodiation in 1 M Na₂SO₄ duringcharging. FIG. 10C depicts the potential-capacity plots of the BiOClelectrode performing dechlorination and the Zn electrode being oxidizedin 70 mM HCl during discharging. For comparison, FIG. 10D shows thepotential-capacity plots of the BiOCl electrode performingdechlorination in 70 mM HCl and the Na₃Ti₂(PO₄)₃ electrode performingdesodiation in 1 M Na₂SO₄ during discharging. All charging anddischarging processes were performed galvanostatically at ±1 mA cm⁻².

DETAILED DESCRIPTION

Bismuth-based, chloride-storage electrodes and rechargeableelectrochemical desalination cells incorporating the chloride-storageelectrodes are provided. Also provided are methods for making theelectrodes and methods for using the desalination cells to removechloride ions from a sample.

The chloride-storage electrodes are composed of bismuth metal. Theelectrodes are able to store chloride ions (Cl⁻) in their bulk byoxidizing Bi to BiOCl in the presence of an oxygen source, such aswater. BiOCl is insoluble in water over a wide pH range and inertagainst water oxidation and oxidatively stable over a wide range ofanodic potentials. As such, bismuth metal electrodes can be used toremove chloride from a variety of aqueous sources. The BiOCl electrodecan be converted back into a Bi electrode by a reduction reaction, whereCl⁻ ions are released into the electrolyte. This reverse reaction allowsfor the repeated use of the Bi electrode for Cl storage and Cl releasevia multiple chlorination and dechlorination cycles.

A chloride-storage electrode can be coupled with a sodium-storageelectrode to construct a rechargeable desalination cell capable ofdesalinating seawater (including ocean water). Some embodiments of thedesalination cells are able to perform a desalination/salination cyclewith a net potential input of only 0.2 V.

The storage of Cl⁻ ions by the Bi electrode is based on the diffusion ofCl⁻ ions in the electrolyte into the Bi lattice to form a newcrystalline phase, BiOCl. Therefore, in order to increase the capacityof Cl storage in the Bi electrode, it is advantageous to produce Bielectrodes as nanocrystalline electrodes with a high porosities, whichprovides them with a high Bi/electrolyte interface area. For example,electrodeposition of bismuth can be used to provide a porousnanocrystalline bismuth foam. The electrodeposition of bismuth onto asupport substrate can be carried out in an electrochemical cell thatincludes the support substrate as a working electrode and furtherincludes a counter electrode. When the support substrate is contactedwith an aqueous electrolyte that includes a bismuth source, such as abismuth halide salt, and an electric potential is applied across theelectrochemical cell, bismuth is plated onto the support substrate viacathodic deposition, while water is simultaneously reduced to H₂ (g).The H₂ bubbles formed at the support substrate surface provide anin-situ generated template around which the bismuth forms a porousmicrostructure. Other methods of fabricating porous bismuth can also beused.

Embodiments of the nanocrystalline bismuth foam are macroporous, havingpore sizes in the range from 1 μm to 100 μm. The walls that define thepores in the nanocrystalline bismuth foams are characterized bycrystalline dendrites—short branched extensions—with cross-sectionaldiameters that are typically less than 100 nm. These dendrites extendoutwardly along the lengths of larger elongated “stems”. As discussed inmore detail in the Example, this porous, nanocrystalline morphology canreduce or prevent pulverization due to the volume expansion andcontraction of the electrode during cell cycling.

In an electrochemical cell, the chloride-storage electrode iselectrically coupled to a second electrode (i.e., a counter electrode),such that oxidation and reduction reactions are carried out at the twoelectrodes when the electrochemical cell is in operation. In order toremove chloride ions from the electrolyte, the bismuth electrode isoxidized to form BiOCl, thereby storing the chloride ions in the bulk ofthe electrode. Suitable chloride ion-containing electrolyte solutionsinclude salinated water from natural bodies of water, such as seawater,and further include chloride ion-containing wastewaters, such asindustrial, municipal, and residential wastewaters.

During the oxidation of the Bi electrode to BiOCl, reduction will occurat the second electrode. The second electrode can be immersed in thesame electrolyte solution as the chloride-storage electrode, as in thecase of an undivided cell, or it can be immersed in a differentelectrolyte solution, as in the case of a divided cell in which the twoelectrolyte solutions are separated by a membrane. Generally, a dividedcell will be preferred if the reaction conditions (e.g., temperatureand/or pH) or the reactants used for the chlorination/dechlorination atthe bismuth electrode would interfere with the complementary redoxreactions occurring at the second electrode, or vice versa.

The nature of the second electrode will depend on the type of reductionreaction to be carried out at that electrode during the operation of theelectrochemical cell. For example, if the electrochemical cell is to beused for desalination, the second electrode will be a sodium-storageelectrode. If the chlorination reaction of the Bi electrode is coupledwith hydrogen (H₂) generation, the second electrode will be a waterreduction electrode, such as a platinum electrode. If the chlorinationreaction of the Bi electrode is coupled with oxygen reduction, thesecond electrode will be an oxygen reduction electrode that reduces theoxygen to, for example, hydroxide ions or hydrogen peroxide.

The BiOCl electrode can be converted back into a Bi electrode by areduction reaction, where BiOCl is reduced to Bi, thereby releasingchloride ions into the electrolyte (e.g., waste water). The pH conditionof the electrolyte may be used (e.g., acidic wastewater) to improve thethermodynamics or kinetics of the dechlorination process and the cycleperformance of the Bi/BiOCl electrode, as illustrated in the Example.While reducing BiOCl to Bi in an electrochemical cell, oxidation occursat the second electrode. The nature of the second electrode will dependon the type of oxidation reaction to be carried out at that electrodeduring the operation of the electrochemical cell. For example, if theelectrochemical cell is to be used for salination, the second electrodewill be a sodium-storage electrode that releases sodium ions duringoxidation. If the dechlorination of BiOCl is coupled with the oxidationof a metal electrode, the second electrode will be a metal electrodewith the desired oxidation potential. Depending on the nature of theoxidation reaction occurring at the second electrode, a divided cell oran undivided cell can be constructed.

A chloride-storage electrode can be coupled with a sodium-storageelectrode to construct a rechargeable desalination cell capable ofdesalinating seawater or brackish water. A sodium-storage electrode isone that can store and release sodium ions (Na^(t)) in the bulk of theelectrode structure by reduction (sodiation) and oxidation (desodiation)reactions. In an electrochemical cell, the chloride-storage electrode iselectrically coupled to the sodium-storage electrode. During thedesalination process of the desalination cell, the chloride storageelectrode undergoes oxidation to store chloride ions, while the sodiumstorage electrode undergoes reduction to store sodium ions. During thesalination process, the chloride storage electrode undergoes reductionto release chloride ions, while the sodium storage electrode undergoesoxidation to release sodium ions.

Depending on the relative redox potentials of the Na-storage electrodeand the Cl-storage electrode, two different types of desalinationelectrochemical cells (Type 1 and Type 2) can result, Type 2 beingillustrated in the Example. The operation of Type 1 and Type 2 cells areillustrated schematically in FIGS. 7A and 7B. If thesodiation/desodiation potential of the Na-storage electrode is morepositive than the chlorination/dechlorination potential of theCl-storage electrode, a Type 1 cell is formed, where desalination occursspontaneously (discharging) and salination requires an energy input(charging). One example of a sodium-storage electrode for a Type 1desalination cell is an MnO₂ electrode, of the kind described in Pastaet al., Nano Lett., 2012, 12(2), 839-843. Other examples ofsodium-storage electrodes for Type 1 electrochemical cells includeNa_(0.44)MnO₂, Na₃V₂(PO₄)₃, NaVPO₄F, CuHCF, and NiHCF electrodes. If thesodiation/desodiation potential of the Na-storage electrode is morenegative than the chlorination/dechlorination of the Cl-storageelectrode, a Type 2 cell is formed, where desalination requires anenergy input (charging) and salination generates an energy output(discharging). The energy released during the discharging processrecovers at least the part of the energy input during the chargingprocess, reducing the net energy required for thedesalination/salination cycle. One example of a sodium-storage electrodefor a Type 2 desalination cell is an NiTi₂(PO₄)₃ electrode, as describedin the Example. Other examples of sodium-storage electrodes for Type 2electrochemical cells include Na₂V₆O₁₆ and Na₂FeP₂O₇ electrodes.

Unlike electrodes that store ions only in a surface double-layer,electrodes that store ions in their bulk can be readily removed from oneelectrolyte solution and transferred to a different electrolytesolution. This is advantageous in embodiments of the desalination cellsin which the sodium storage reaction at the second electrode iscompleted before the chloride storage reaction at the Bi electrode iscompleted. In this situation, the second electrode can be replaced witha different electrode (a “replacement electrode”) that carries out adifferent reduction reaction, as the oxidation of Bi to BiOCl continuesto completely remove chloride ions in the electrolyte. This newreduction reaction can be one that is carried out in the sameelectrolyte that is in contact with the chloride-storage electrode, orit can be one that is carried out in a separate electrolyte solution.This provides a flexible approach that allows the user to select areduction reaction that minimizes the energy input needed todechlorinate the electrolyte solution and/or that allows the user togenerate a desired reduction product. By way of illustration, aNaTi₂(PO₄)₃ sodium-storage electrode in a Type II desalination cellcould be replaced by a hydrogen generation electrode (i.e., an electrodeat which water is reduced to H₂(g)) once the NaTi₂(PO₄)₃ became fullysodiated, as discussed in the Example. Alternatively, electrodes thatreduce O₂, or toxic metal ions, such as Pb or Hg ions, can be used.

Similarly, if the oxidation of the second electrode (i.e., desodiationof the sodium-storage electrode) during the salination cycle of thedesalination cell is completed before the complete dechlorination of theBiOCl in the chloride-storage electrode, the dechlorination of BiOCl canbe completed by replacing the second electrode with a differentelectrode (a “replacement electrode”) that carries out a differentoxidation reaction until the dechlorination of the BiOCl in thechloride-storage electrode is completed. Here, again, the new oxidationreaction can be carried out in the same electrolyte solution that is incontact with the chloride-storage electrode or a separate electrolytesolution, and can be strategically selected to minimize the energy inputneeded to operate the cell. For example, in a Type 2 desalination cell,the sodium-storage electrode can be replaced by an electrode thatoxidizes at a more negative potential than the sodium-storage electrodethat it replaces. By way of illustration, a NaTi₂(PO₄)₃ sodium-storageelectrode in a Type II desalination cell could be replaced by a zincelectrode once the Na₃Ti₂(PO₄)₃ has been fully desodiated, as discussedin the Example. Alternatively, electrodes that remove toxic pollutantsfrom the electrolyte solution via oxidation can be used. For example, anelectrode that oxidizes organic molecules, such as dye molecules orbenzene to CO₂ could be used. Or, an electrode that oxidizes Cl— toCl₂(g) can be employed.

EXAMPLE

This example reports a nanocrystalline Bi foam electrode as an efficientand high capacity Cl-storage electrode, which stores Cl⁻ ions in theform of BiOCl. Because both Bi and BiOCl are insoluble in saline aqueousconditions in a wide pH and potential range, a sustainable use of Bi andBiOCl electrodes in seawater is possible. The performance of Bi andBiOCl electrodes was investigated for their half-cell reactions (i.e.,chlorination of Bi and dechlorination of BiOCl), and then a desalinationcell was constructed where the Bi electrode is paired with NaTi₂(PO₄)₃as a Na-storage electrode. Since both Bi and NaTi₂(PO₄)₃ electrodesstore salt ions in the bulk of the electrodes, the Bi/NaTi₂(PO₄)₃ cellhas the capacity to desalinate seawater. By optimizing the desalinationand salination conditions, it was demonstrated that the Bi/NaTi₂(PO₄)₃cell achieved desalination with an potential input of 0.75 V andsalination with a potential output of 0.55 V when the desalination andsalination processes were performed galvanostatically at ±1 mA cm⁻².This means that the Bi/NaTi₂(PO₄)₃ cell can perform adesalination/salination cycle with a net potential input of only 0.20 V.

The Pourbaix diagram of Bi, which shows the various species of Bipresent as functions of pH and potential in a 0.6 M Cl⁻ solution (theconcentration of Cl⁻ in the seawater) was constructed and compared withthat in pure water in FIG. 1(A) and FIG. 1(B) (M. Pourbaix, Atlas ofelectrochemical equilibria in aqueous solutions (NACE, Houston Tex., ed.2, 1974), pp. 1-644.) In pure water, the oxidation product of Bi metalis either Bi₂O₃, if the pH was above 4.5, or Bi³⁺ ions (e.g. Bi³⁺ orBi₆(OH)₁₂ ⁶⁺), if the pH is below pH 4.5. However when Cl⁻ ions arepresent, oxidation of Bi resulted in the formation of insoluble BiOCl ina wide range of pH conditions (0<pH<10.5).

Unlike CDI, the storage of Cl⁻ ions by the Bi electrode will require thediffusion of Cl⁻ ions into the Bi lattice to form a new crystallinephase, BiOCl. In this case, incorporating a porous structure anddecreasing the size of the Bi crystals can be highly advantageous asthese can increase the interfacial area and facilitate Cl⁻ diffusioninto the Bi lattice, maximizing the amount of Bi that can be convertedto BiOCl. Therefore, a high surface area nanocrystalline Bi foamelectrode was prepared by electrodeposition to examine the use of Bi asa Cl-storage electrode for use in a desalination cell.

The foam structure of Bi shown in FIG. 2A was obtained by depositing Biwhile concurrently reducing water to H₂. The H₂ bubbles formed on theworking electrode served as an in-situ generated template to depositmicroporous Bi electrodes. The high magnification scanning electronmicroscopy (SEM) image shows that the wall of the foam structure iscomposed of nanoscale needle-like dendrites, contributing further to theincrease in the surface area of the electrode (FIG. 2(B)).

The electrochemical behavior of the resulting Bi electrode was firstinvestigated in a 0.6 M NaCl solution, which mimics the NaClconcentration in seawater, using linear sweep voltammetry (LSV). Theelectrochemical reaction involving the conversion between Bi and BiOClis shown in eq. 1. If this reaction is conducted in a 0.6 M NaClsolution (pH 6.4), the equilibrium reduction potential is calculated tobe −0.088 V vs. SHE using the Nernst equation, which is equivalent to−0.285 V vs. the Ag/AgCl (in 4 M KCl) reference electrode (RE).BiOCl+2H⁺+3e ⁻⇄Bi+Cl⁻+H₂O E^(o)=0.160 V vs. SHE  (eq. 1)

An LSV scan to the positive direction (FIG. 3(A)) showed the onset ofanodic current at approximately −0.15 V vs. Ag/AgCl and an anodiccurrent peak centered at −0.01 V vs. Ag/AgCl. After the oxidation of Bito BiOCl, no other oxidation reaction such as water oxidation wasobserved on the BiOCl surface when the potential was scanned to 2.5 Vvs. Ag/AgCl (=3.1 V vs. RHE). Since the water oxidation reaction cancompete with the Cl⁻ storage reaction, the fact that BiOCl wascompletely inert for water oxidation and was oxidatively stable over awide window of anodic potentials is a great advantage for the use of Bifor the desalination process. An LSV scan to the negative direction(FIG. 3(A)) shows the onset of cathodic current at about −0.45 V vs.Ag/AgCl and a cathodic current peak centered at −1.27 V vs. Ag/AgCl,which is due to the reduction of BiOCl to Bi. After the reduction ofBiOCl, an additional cathodic wave, which is due to the reduction ofwater, follows.

Comparing the anodic and cathodic peak positions with the equilibriumredox potential of Bi/BiOCl, the reduction kinetics of BiOCl to Biappear to be much more sluggish than the oxidation kinetics of Bi toBiOCl. Considering that redox reactions involving the formation of solidphases are typically much slower than those involved with only solutionspecies, the oxidation kinetics of Bi to BiOCl, requiring only anoverpotential of ˜100 mV to initiate the oxidation, appears to be quitefast.

The capacities for Bi to store Cl⁻ ions and for BiOCl to release Cl⁻ions were investigated by applying a constant current density (1 mA cm⁻²for the oxidation of Bi to BiOCl and −1 mA cm⁻² for the reduction ofBiOCl to Bi). The potential changes against the Ag/AgCl RE during theseconversion processes were recorded as a function of capacity (FIG.3(B)). When the oxidation of Bi was initiated, the potential rapidlyincreased from its open circuit potential and plateaued at −0.08 V vs.Ag/AgCl. When the oxidation reaction was terminated with a cut-offpotential of 0.5 V, the total capacity was determined to be 0.2351 mAhcm⁻². The Bi electrode used in this study contained 1.25±0.21 mg cm⁻² ofBi, and, therefore, the full conversion of Bi to BiOCl shouldtheoretically require 0.4810 mAh cm⁻². Comparing the theoretical andobserved capacities, 48.9% of the Bi in the Bi electrode was estimatedto be electrochemically active. With 48.9% of the Bi beingelectrochemically active, the amount of Cl that can be stored per 1 g ofBi is calculated to be 82.96 mg. This result is impressive consideringthat the amount of Cl that can be stored by the CDI system using aporous carbon electrode is estimated to be 16.38 mg per 1 g of carbon(calculation details discussed below). (J. Lee, S. Kim, C. Kim, J. Yoon,Hybrid capacitive deionization to enhance the desalination performanceof capacitive techniques, Energy Environ. Sci. 7, 3683-3689 (2014).) If100% Bi becomes electrochemically active by further morphologyoptimizations, the amount of Cl that can be stored per 1 g of Bi willincrease to 169.6 mg.

The X-ray diffraction patterns of the as-deposited Bi before and afteroxidation at 1 mA cm⁻² are shown in FIG. 4A, panels (a)-(e). Theas-deposited Bi electrode was crystalline and showed peaks generatedonly by Bi. After oxidation, the intensities of the Bi diffraction peakswere diminished and new peaks, which can be all assigned to peaks forBiOCl, appeared confirming the ability for Bi to store Cl⁻ ascrystalline BiOCl.

When reduction of BiOCl was performed at −1 mA cm⁻², the potentialdecreased from its open circuit value and plateaued at around −1.15V-−1.2 V vs. Ag/AgCl (FIG. 3(B)). When −1.5 V vs. Ag/AgCl was used asthe cut-off potential, the capacity for reduction was determined to be0.2685 mAh cm⁻², which is slightly higher than that of oxidation. Sincethe amount of Cl⁻ released cannot exceed the amount of Cl⁻ stored, theobserved increase in capacity should be due to the water reductionreaction that occurred simultaneously as a minor side reaction.

When the reduced electrode was examined by XRD, the peaks assigned toBiOCl completely vanished, and the crystallinity of Bi was fullyrecovered (FIGS. 4A and 4B). The same behavior was observed for anadditional redox cycle, indicating that crystalline BiOCl and Bi can beformed repeatedly during the Cl storage/release cycles. The elementalanalysis performed on the Bi and BiOCl electrodes during two redoxcycles using energy-dispersive X-ray spectroscopy (EDS) (Table 1) agreeswell with the XRD results.

TABLE 1 The atomic ratios of the Bi electrode during the first twochlorination/dechlorination cycles obtained by EDS analysis Filmcomposition Conversion state Atomic ratio of Bi:Cl As-deposited 1:0.043(±0.005) After 1^(st) Chlorination 1:0.976 (±0.011) After 1^(st)Dechlorination 1:0.017 (±0.005) After 2^(nd) Chlorination 1:0.939(±0.013) After 2^(nd) Dechlorination 1:0.020 (±0.005)

The morphology of the BiOCl electrode obtained by oxidizing Bi at 1 mAcm⁻² is shown in FIG. 2(C). After oxidation, needle-like Bi dendriteswere converted to thin two-dimensional sheets of BiOCl. When BiOCl wasreduced back to Bi, the needle-like Bi dendrites reappeared (FIG. 2(D)).The same morphological change was repeated for an additional redox cycle(FIGS. 2(E) and 2(F). The nanocrystalline dendritic Bi morphology thatprovides ample space between the needle-like nanocrystals of Bi appearsto be highly beneficial for the observed cycling ability as it offerssufficient room for individual Bi crystals to become BiOCl sheets, whichinvolves an increase in volume by 158% (calculation details discussedbelow).

The cyclability of the Bi/BiOCl electrode for Cl⁻ storage and releasewas evaluated by applying ±2 mA cm⁻² for 200 cycles. Cut-off potentialsof 0.5 V and 1.4 V vs. Ag/AgCl were used for the oxidation andreduction, respectively. FIG. 3(C) shows the capacities measured for Cl⁻storage versus cycle number. The initial and the final capacities forCl⁻ ion capture were 0.2216 mAh cm⁻² and 0.1346 mAh cm⁻², respectively,corresponding to the retention of 60.77% of the initial capacity after200 cycles. It was postulated that the ˜40% reduction in the capacitanceis most likely due to the mechanical loss of Bi and BiOCl caused bypulverization. This means that the volume change involved with theconversion of Bi and BiOCl resulted in the disintegration of a portionof the nanoparticles/nano plates from the electrode. In this case,intentionally limiting the amount of Bi that is converted to BiOCl sothat the core region of the Bi electrode serving as the “stem” of thedendritic Bi remains as Bi can help to enhance the cyclability of theelectrode by retaining the connection between the crystals. Thispostulation was tested by repeating the cycling test while limiting thecapacity for oxidation to 0.1756 mA cm⁻², which is 80% of the initialcapacity of the Cl storage shown in FIG. 3(C). (The potential-capacityplots measured for the first three Cl storage/release cycles can befound in FIGS. 5(A) and 5(B).) The result shows that indeed the initialcapacity of 0.1756 mA cm⁻² was maintained over 200 cycles without anyloss, demonstrating that with further morphology optimization thatminimizes the pulverization problem, excellent cyclability for highercapacities should be possible.

To demonstrate the use of the Bi electrode for the overall desalinationprocess, the Bi electrode needed to be coupled with a Na-storageelectrode. Among a few materials reported in the field of aqueousrechargeable Na batteries, NASICON-type NaTi₂(PO₄)₃ was selected forthis study due to its high specific storage capacity of 133 mA h g⁻¹ andwell-defined and relatively low redox potential of Ti⁴⁺/Ti³⁺ atapproximately −0.8 V (Ag/AgCl), which is coupled with thesodiation/desodiation processes as shown in eq. 2.NaTi₂(PO₄)₃+2Na⁺+2e ⁻↔Na₃Ti₂(PO₄)₃  (eq. 2)

NaTi₂(PO₄)₃ electrodes used in this study were prepared by a sol-gelmethod.

Since it is reported that NaTi₂(PO₄)₃ typically suffers from the lowintrinsic electrical conductivity, carbon was coated on NaTi₂(PO₄)₃through pyrolysis of sucrose. The XRD pattern for the pristineNaTi₂(PO₄)₃ powder confirmed the synthesis of well-crystallizedNASICON-type NaTi₂(PO₄)₃ (FIG. 6(A)).

The sodiation/desodiation profiles of NaTi₂(PO₄)₃ in a 1 M Na₂SO₄solution, which is typically used for aqueous rechargeable Na batteries,at a current density of ±2 mA cm⁻² between −1.0 and −0.4 V vs. Ag/AgCl,are shown in FIG. 6(B). Upon reduction, a plateau at −0.83 V vs. Ag/AgClwas observed, corresponding to the sodiation of NaTi₂(PO₄)₃ accompaniedby the reduction of Ti⁴⁺ to Ti³⁺. Upon the following oxidation, aplateau at −0.78 V vs. Ag/AgCl was achieved, which corresponds todesodiation accompanied by the oxidation of Ti³⁺ to Ti⁴⁺. The observedbehaviors agree well with the typical voltage profiles of NaTi₂(PO₄)₃.(H. Kim, J. Hong, K.-Y. Park, H. Kim, S.-W. Kim, K. Kang, AqueousRechargeable Li and Na Ion Batteries, Chem. Rev. 114, 11788-11827(2014); S. I. Park, I. Gocheva, S. Okada, J. Yamaki, Electrochemicalproperties of NaTi₂(PO₄)₃ anode for rechargeable aqueous sodium-ionbatteries, J. Electrochem. Soc. 158, A1067-A1070 (2011); Z. Li, D.Young, K. Xiang, W. C. Carter, Y.-M. Chiang, Towards high power highenergy aqueous sodium-ion batteries: The NaTi₂(PO₄)₃/Na_(0.44)MnO₂system, Adv. Energy Mater. 3, 290-294 (2013).) The NaTi₂(PO₄)₃ electrodealso showed excellent cycle stability with good capacity retention andCoulombic efficiency (FIG. 6(C)).

After the synthesis and characterization of the NaTi₂(PO₄)₃ electrode, adesalination cell composed of a Bi electrode and a NaTi₂(PO₄)₃ electrodewas constructed. Depending on the relative redox potentials of theNa-storage electrode and the Cl-storage electrode, two different typesof desalination cells could have resulted. If the sodiation/desodiationpotential of the Na-storage electrode was more positive than thechlorination/dechlorination potential of the Cl-storage electrode,desalination would occur spontaneously (discharging), while salinationwould require an energy input (charging) (FIG. 7(A)), referred to asType 1 desalination cell. The Ag/MnO₂ cell reported previously belongsto this type (9). The energy released during desalination by the Type 1cell could then be utilized to perform additional work. If thesodiation/desodiation potential of the Na-storage electrode were morenegative than the chlorination/dechlorination of the Cl-storageelectrode, desalination would require an energy input (charging), whilesalination would generate an energy output (discharging), which is shownas Type 2 in FIG. 7(B). Once the electrochemical cell was charged bydesalination, the energy released during salination could be utilizedfor any desired work. The Bi/NaTi₂(PO₄)₃ cell constructed in this studywas the first example of a Type 2 desalination cell. The overalldesalination reaction of the Bi/NaTi₂(PO₄)₃ cell is shown in eq. 3.3/2NaTi₂(PO₄)₃+Bi+3Na⁺+Cl⁻+H₂O→3/2Na₃Ti₂(PO₄)₃+BiOCl+2H⁺  (eq. 3)

The desalination performance of the Bi/NaTi₂(PO₄)₃ cell was firstexamined in an undivided cell containing a 0.6 M NaCl solution at aconstant current density of 1 mA cm-². While the cell potential betweenthe Bi electrode and the NaTi₂(PO₄)₃ electrode was measured, theirindividual potentials vs. Ag/AgCl were also recorded, which is usefulfor understanding the cell performance. The individual potentialprofiles showed that the anode reaction (Cl-storage) occurred at a morepositive potential than the cathode reaction (Na-storage) by ˜0.75 V(FIG. 8A), meaning that the overall desalination process required anaverage potential input of 0.75 V. (For the overall reaction to generatean energy output, the cathode reaction should occur at a more positivepotential than the anode reaction.)

FIG. 8(B) shows the individual potential profiles of the BiOCl andNa₃Ti₂(PO₄)₃ electrodes measured vs. Ag/AgCl during the salinationprocess in the same solution at 1 mA cm⁻². Thermodynamically, since thedesalination process is non-spontaneous, its reverse reaction, thesalination process, should be spontaneous. However, as discussedearlier, the reduction of BiOCl to Bi in 0.6 M NaCl is kinetically verysluggish and requires a significant over-potential. As a result, thereduction of BiOCl occurs at a more negative potential than theoxidation of Na₃Ti₂(PO₄)₃, thus again requiring an energy input for thesalination process.

In order to recover energy during the salination process, thepossibility of improving the reduction kinetics of BiOCl by changingsolution compositions was examined. It should be noted that whileseawater is used as the feed water for the desalination process forseawater desalination, solutions other than seawater (e.g., waste water)can be used for the salination process if they can improve thethermodynamics or kinetics of the salination process and the overallcycle performance. For the CDI systems, which require electrodes toremain immersed in solution between desalination and the salination tonot lose salt ions stored in the electrical double layer, alteringsolutions for the desalination and salination processes may adddifficulty to preventing contamination of the desalinated water.However, electrodes used in desalination cells, which store the saltions in the bulk of the electrode, can be lifted out of the desalinatedwater after desalination and immersed into a different solution forsalination without affecting the salt storage. This will provide moreflexibility in the design and operation of desalination/salinationsystems.

It was discovered that the reduction kinetics of BiOCl could be improvedsignificantly in acidic media (pH<2). In this study, a 70 mM HClsolution (pH 1.15), which can be replaced with acidic wastewater inpractice, was chosen as an example solution to carry out reduction ofBiOCl with improved kinetics (FIG. 9(B)). The LSV for the reduction ofBiOCl in 006 M NaCl (pH=6.4) is shown in FIG. 9(A) for comparison. Theuse of acidic solution, however, is not favorable for desodiation ofNa₃Ti₂(PO₄)₃ due to its self-discharge reaction where Na₃Ti₂(PO₄)₃ isspontaneously oxidized by reducing water (16). Therefore, salination wasperformed in a divided cell where the reduction of BiOCl was performedin a 70 mM HCl solution, while the oxidation of Na₃Ti₂(PO₄)₃ wasperformed in a 1 M Na₂SO₄ solution, which is the solution used foraqueous sodium ion batteries, thus ensuring the best cycle performance.A Nafion membrane was used as a divider. The operating condition for theBi/NaTi₂(PO₄)₃ cell is summarized in FIG. 7C.

FIG. 8(C) shows the potential profiles of the BiOCl and the Na₃Ti₂(PO₄)₃electrodes for salination in a divided cell. The desodiation potentialof the Na₃Ti₂(PO₄)₃ in 1 M Na₂SO₄ vs. Ag/AgCl is comparable to that in0.6 M NaCl. However, the potential required for dechlorination of BiOClis changed from −1.25 V to −0.15 V when a 0.6 M NaCl solution isreplaced with a 70 mM HCl solution. The difference in the equilibriumpotential for the reduction of BiOCl to Bi in these two solutions isonly 222 mV. This means that the over-potential to achieve 1 mA cm⁻² isreduced by 878 mV by using a 70 mM HCl solution. As a result,dechlorination occurs at a more positive potential than desodiation inthe divided cell, and the overall salination process can generate anaverage potential output of 0.55 V.

The cell voltage change of the Bi/NaTi₂(PO₄)₃ cell as a function of thecapacity during the first three desalination/salination cycles at ±1 mAcm⁻² is shown in FIG. 8(D). As expected from the results shown in FIG.8(C), the Bi/NaTi₂(PO₄)₃ system requires an average potential input of0.75 V for desalination (equivalent to 0.175 mWh cm⁻²) and generates anaverage cell voltage of 0.55 V for salination (equivalent to 0.147 mWhcm⁻²). This means that only a net potential of 0.2 V is required perdesalination/salination cycle at ±1 mA cm⁻² (equivalent to 0.028 mWhcm⁻²). The Bi/NaTi₂(PO₄)₃ cell showed excellent cycle performance withnegligible changes for both the charging and discharging capacitiesafter 30 cycles (FIG. 8(E)). The minor fluctuation in capacity shown inthe cycle test is due to our procedure that involves manually liftingand immersing the Bi and NaTi₂(PO₄)₃ electrodes in two differentsolutions repeatedly. The reason the discharging capacity is slightlyhigher than the charging capacity is again due to the H₂ evolutionoccurring at the BiOCl electrode as a side reaction during salination.

It should be noted that the molar ratio of the Na⁺ and Cl⁻ ions storedin the Bi/NaTi₂(PO₄)₃ cell during desalination is not 1:1 but 3:1 (eq.3). This is because while the NaTi₂(PO₄)₃ electrode stores one Na perelectron, the Bi electrode stores one Cl per three electrons (eq. 1). Ina practical sense, this means that during desalination when Na⁺ in theseawater is completely removed, 2/3 of the Cl⁻ would still remain in thesolution. However, this issue can be resolved by replacing a fullycharged Na₃Ti₂(PO₄)₃ electrode with an electrode that can perform waterreduction to generate H_(z), a valuable fuel, while Bi finishes removingthe remaining Cl⁻ ions. The evolution of H₂ will also remove H⁺introduced to the solution during the oxidation of Bi to BiOCl (eq. 1),bringing the pH of the desalinated water back to a neutral range.Preliminary results using Pt as the hydrogen evolution electrode, whichcan be replaced with an inexpensive alternative for practical celloperation, showed that the potential required to pair the chlorinationof Bi and H₂ evolution (˜0.5 V) was smaller than the potential requiredfor the chlorination of Bi and sodiation of NaTi₂(PO₄)₃ (FIGS. 10(A) and10(B)). This potential was also much smaller than the potential requiredto produce H₂ by water splitting (>1.23 V). In the same manner, ifdesodiation of Na₃Ti₂(PO₄)₃ were completed before dechlorination ofBiOCl during salination, dechlorination of BiOCl could be completed byintroducing a sacrificial anode reaction, such as oxidation of zinc,which occurs at a more negative potential than desodiation ofNa₃Ti₂(PO₄)₃ and thus further increases the potential output (FIGS.10(C) and 10(D)). The zinc ions released into the solution could berecovered by electrochemical deposition of zinc paired with oxidation ofCl⁻ ions to Cl₂, for example. In other words, thedesalination/salination cell could be strategically integrated withvarious electrochemical processes to maximize the useful outcomes (e.g.,fresh water, energy output, H₂, and Cl₂) while minimizing the energyinput required for the overall process.

In summary, Bi was demonstrated to be a practical and sustainable highcapacity Cl-storage electrode through the use of its conversion toBiOCl. When coupled with NaTi₂(PO₄)₃, a new type of a desalination cellthat charges by desalination and discharges by salination wasconstructed, which enabled the desalination/salination cycle at ±1 mAcm⁻² with a net average potential input of only 0.2 V.

Materials and Methods

Synthesis and Characterization.

The Bi electrodes used in this study were prepared by electrodeposition.A three-electrode setup composed of a Ti sheet as the working electrode,a Pt sheet as the counter electrode, and a saturated calomel electrode(SCE) as the reference electrode were used for deposition in anundivided cell. An aqueous solution containing 14 mM BiCl₃ (Alfa Aesar),1.4 M HCl (Sigma-Aldrich, 37%), and 2.5 g/L polyethylene glycol (PEG)6000 (USB Corporation) was used as a plating solution. Cathodicdeposition (Bi³⁺+3e⁻→Bi, E^(o)=0.286 V vs. SHE) was carried outpotentiostatically at −2.6 V vs. SCE for 2 min. The distance between theworking electrode and the counter electrode was 2 cm, and the solutionwas magnetically stirred at 300 rpm. The average current density wasapproximately −800 mA cm⁻². After deposition, Bi electrodes were rinsedwith distilled water and dried in air. Since vigorous water reductionoccurred concurrently with Bi electrodeposition, the amount of Bideposited could not be determined by the total charge passed assuming100% Faradaic efficiency for Bi deposition. Therefore, the actualloading of Bi was measured from the weight difference of the workingelectrode before and after the electrodeposition of Bi, which was1.25±0.21 mg cm⁻².

NaTi₂(PO₄)₃ was prepared by a sol-gel method following the procedurereported in a previous study. (S. I. Park, I. Gocheva, S. Okada, J.-i.Yamaki, Electrochemical properties of NaTi₂(PO₄)₃ anode for rechargeableaqueous sodium-ion batteries, J. Electrochem. Soc. 158, A1067-A1070(2011).) A 0.02 M titanium butoxide solution was first prepared bydissolving titanium butoxide (C₁₆H₃₆O₄Ti, Sigma-Aldrich) in a solutioncomposed of 280 mL hydrogen peroxide (H₂O₂, 30% solution) and 120 mLammonium hydroxide (NH₄OH, 28-30% NH₃ basis) while stirring. Citric acidwas added to this solution to achieve a 0.04 M citric acid solution.Stoichiometric amounts of ammonium phosphate monobasic (NH₄H₂PO₄,Sigma-Aldrich) and sodium carbonate (Na₂CO₃, Macron) to react with Ti inthe titanium butoxide solution to form NaTi₂(PO₄)₃ were calculated anddissolved in 80 mL DI water and in 100 mL nitric acid (HNO₃, 70%),respectively. Then, these solutions were combined with the titaniumbutoxide solution. The combined solution was stirred at 80° C. for 10 h,and then dried at 140° C. for 2 h to obtain a yellow powder. Theresulting powder was annealed at 800° C. for 12 h (ramping rate, 2° C.min⁻¹) in air to obtain highly crystalline NaTi₂(PO₄)₃ powders. Thecarbon coating of NaTi₂(PO₄)₃ to form NaTi₂(PO₄)₃/C composites wasachieved via ethylene glycol (EG) assisted pyrolysis of sucrose. Forthis treatment, 25 mg of sucrose (C₁₂H₂₂O₁₁, Sigma-Aldrich) wasdissolved in a mixture of 2 mL DI water and 0.5 mL EG, and then 0.5 g ofthe synthesized NaTi₂(PO₄)₃ powder was dispersed in the solution byultrasonication for 30 min. After drying at 40° C. for 30 min, theresulting gray powder was annealed at 700° C. (ramping rate, 5° C.min⁻¹) in an N₂ atmosphere for 2 hours. To fabricate the NaTi₂(PO₄)₃electrodes, 70 wt. % NaTi₂(PO₄)₃/C powder, 20 wt. % carbon black (AlfaAesar) and 10 wt. % polytetrafluoroethylene (Sigma-Aldrich) were mixedwith Nmethyl-2-pyrrolidone (NMP). After thorough mixing, the paste waskneaded and pressed to prepare sheet-type electrodes with a thickness of˜120 μm. The fabricated electrodes were kept at 80° C. for 10 h toremove the remaining solvent. After drying, the sheet was punched into adisc shape with a 1 cm² area, then pressed onto the graphite currentcollector.

The surface morphology and crystal structures of the materials wereexamined by scanning electron microscopy (SEM) (a LEO 1530 at anaccelerating voltage of 5 kV) and powder X-ray diffraction (XRD) (BrokerD8 Advanced PXRD, Ni-filtered Cu Kα radiation, λ=1.5418 Å) at roomtemperature. The composition of the electrode was analyzed byenergy-dispersive X-ray spectroscopy (EDS) (Noran System Seven, ThermoFisher) at an accelerating voltage of 15 kV. To investigate the phaseand morphological changes of the electrodes after oxidation andreduction, ex-situ SEM analysis and ex-situ XRD analysis were performed.

Electrochemical Experiments.

Linear sweep voltammetry (LSV) and galvanostatic charging/dischargingtests for half-cell reactions (i.e., chlorination/dechlorination of Biand sodiation and desodiation of NaTi₂(PO₄)₃) were performed using athree-electrode setup with an undivided cell. Either a Bi electrode oran NaTi₂(PO₄)₃ electrode was used as the working electrode with a Ptsheet as the counter electrode and a Ag/AgCl (4M KCl) electrode as thereference electrode. A scan rate of 5 mV s⁻¹ was used.

The desalination and salination performances of the Bi/NaTi₂(PO₄)₃ cellwere examined in an undivided cell containing a 0.6 M NaCl (Macron)solution. The salination performance was also examined in a divided cellcontaining a 70 mM HCl solution (pH 1.15) for the BiOCl electrode and a1 M Na₂SO₄ solution for the Na₃Ti₂(PO₄)₃ electrode. The two compartmentswere separated by a Nafion membrane (Nafion N-117, Fuel cell store).

The desalination and the salination performances of the cells weretested galvanostatically at ±1 mA cm⁻². During these processes, inaddition to measuring the cell voltage between the Bi and theNaTi₂(PO₄)₃ electrodes as a function of capacity, the individualpotentials of the Bi and the NaTi₂(PO₄)₃ electrodes against the Ag/AgClreference electrode were recorded. (When a divided cell was used, thereference electrode was placed in a 1 M Na₂SO₄ solution.) The chargingprocess was performed until the cell voltage reached 1.1 V, and thedischarging process was performed until the cell voltage reached 0 V.When different solutions were used for the charging/dischargingprocesses, after the charging was finished, the Bi electrode andNaTi₂(PO₄)₃ electrode were manually lifted from an undivided cell,rinsed with DI water, and brought into a divided cell for thedischarging process.

As mentioned above, the discharging capacity for the Bi electrode isalways higher than its charging capacity due to water reduction thatoccurs as a side reaction during discharging. Therefore, for the cycletest of the Bi/NaTi₂(PO₄)₃ cell, it was necessary to use a NaTi₂(PO₄)₃electrode that has a much higher charging/discharging capacity than thatof Bi so that the discharging capacity due to the water reduction at theBiOCl electrode can be compensated for by the excess dischargingcapacity of Na₃Ti₂(PO₄)₃. The initial charging capacity of theNaTi₂(PO₄)₃ electrode used in this study was 1.1 mAh cm⁻² while theinitial charging capacity of the Bi electrode was 0.23 mAh cm⁻². Beforethe cycle test shown in FIG. 8(E), the NaTi₂(PO₄)₃ electrode was fullysodiated in 1 M Na₂SO₄ and the Bi electrode was fully chlorinated in 0.6M NaCl separately. The two electrodes were then discharged together in adivided cell. The Na₃Ti₂(PO₄)₃ electrode discharged together with BiOClstill contained a significant amount of Na₃Ti₂(PO₄)₃, which can be usedto compensate for over 100% Coulombic efficiency of the Bi electrodeduring the cycle test. After this initial charge/discharge process, theBi/NaTi₂(PO₄)₃ cell was used for the cycle test shown in FIG. 8(E).During the cycle test, when necessary, NaTi₂(PO₄)₃ was additionallycharged to compensate for over 100% Coulombic efficiency of the Bielectrode for the remaining cycle test.

Calculation Details

Construction of Pourbaix Diagram for the Bi—Cl—H₂O System.

Using the ΔG_(f)° values for all soluble and insoluble species for theBi—Cl—H₂O system, equilibrium potentials for all possibleelectrochemical reactions, as well as the solubilities of varioussolution species at 25° C., were calculated. Using this data, thePourbaix diagram was constructed using Chesta software (FIGS. 1(A) and1(B)). The activity coefficients for all species were assumed to be 1.

Volume Expansion of Bi During Cl⁻ Storage Process (Phase Transition toBiOCl):

Unit cell volume of rhombohedral Bi:

(a=4.54 Å, b=4.54 Å, c=11.86 Å, α=90°, β=90° γ=120°)V _(unit cell)=4.54 Å*4.54 Å*√{square root over (3)}/2*11.86 Å=211.703Å³Since there are 6 Bi atoms per unit cell,V _(unit cell)/atom_(Bi)=211.703 Å³/6=35.283 Å³Unit cell volume of tetragonal BiOCl:

(a=3.89 Å, b=3.89 Å, c=7.37 Å, α=90°, β=90° γ=90°)V _(unit cell)=3.89 Å*3.89 Å*7.37 Å=111.524 Å³Since there are 2 Bi atoms per unit cell,V _(unit cell)/atom_(Bi)=111.524 Å³/2=55.762 Å³Therefore, the volume change of Bi during a phase transition to BiOCl is158%.

Calculation of Equilibrium Redox Potentials for Bi/BiOCl:BiOCl+2H⁺+3e ⁻⇄Bi+Cl⁻+H₂OStandard reduction potential:E°=0.160 V vs. SHE, which is equivalent to −0.0372 V vs. Ag/AgCl (4MKCl)Equilibrium potential in 0.6 M NaCl (pH 6.4):

$\begin{matrix}{E = {{E\;{^\circ}} - {\frac{0.0591\mspace{14mu} V}{n}\log\mspace{14mu} a_{{Cl}^{-}}} + {2\left( \frac{0.0591\mspace{14mu} V}{n} \right){\log\left\lbrack H^{+} \right\rbrack}}}} \\{= {{E\;{^\circ}} - {0.0197\mspace{14mu}{V\left( {\log\mspace{14mu} a_{{Cl}^{-}}} \right)}} - {0.0394\mspace{14mu}{V({pH})}}}} \\{= {{E\;{^\circ}} - {0.0197\mspace{14mu} V\mspace{14mu}\log\mspace{14mu}(0.6)} - {0.0394\mspace{14mu}{V(6.4)}}}} \\{= {{0.160\mspace{14mu} V} + {0.00437\mspace{14mu} V} - {0.25216\mspace{14mu} V}}} \\{= {{- 0.088}\mspace{14mu} V\mspace{14mu}{{vs}.\mspace{14mu}{SHE}}}}\end{matrix}$ or   − 0.285  V  vs.  Ag/AgClEquilibrium potential in 70 mM HCl (pH 1.15):

$\begin{matrix}{E = {{E\;{^\circ}} - {\frac{0.0591\mspace{14mu} V}{n}\log\mspace{14mu} a_{{Cl}^{-}}} + {2\left( \frac{0.0591\mspace{14mu} V}{n} \right){\log\left\lbrack H^{+} \right\rbrack}}}} \\{= {{E\;{^\circ}} - {0.0197\mspace{14mu}{V\left( {\log\mspace{14mu} a_{{Cl}^{-}}} \right)}} - {0.0394\mspace{14mu}{V({pH})}}}} \\{= {{E\;{^\circ}} - {0.0197\mspace{14mu} V\mspace{14mu}\log\mspace{14mu}(0.07)} - {0.0394\mspace{14mu}{V(1.15)}}}} \\{= {{0.160\mspace{14mu} V} + {0.02275\mspace{14mu} V} - {0.04531\mspace{14mu} V}}} \\{= {{- 0.13744}\mspace{14mu} V\mspace{14mu}{{vs}.\mspace{14mu}{SHE}}}}\end{matrix}$ or   − 0.0596  V  vs.  Ag/AgCl

Capacity of Bi for Cl Storage.

When assuming complete conversion of Bi to BiOCl by the followingreaction:Bi+Cl⁻+H₂O→BiOCl+2H⁺+3e ⁻since one mole of Cl (MM=35.453 g/mol) can be stored per one mole of Bi(MM=208.9804 g/mol), the theoretical capacity of Cl storage in 1 g of Biis calculated to be 169.6 mg/g_(Bi). Storing one mole of Cl in one molof Bi requires three moles of e⁻, which is equivalent to 289, 455 C.Since Bi electrodes used in this study contain 1.25 mg of Bi (5.98×10⁻⁶mol), the total charge necessary to completely convert Bi to BiOCl iscalculated to be:Theoretical charge required to convert 1.25 mg of Bi to BiOCl=5.98×10⁻⁶mol_(Bi)×289,455 C/mol_(Bi)=1.73 CHowever, the total charge experimentally observed to convert 1.25 mg ofBi to BiOCl was 0.2351 mAh, which is equivalent to 0.846 C. This meansthat during chlorination, only 48.9% of the Bi (0.611 mg) in the Bielectrode is electrochemically active and was converted to BiOCl.Therefore, although the theoretical capacity for Cl that can be storedin 1 g of Bi is 169.6 mg/g_(Bi), the experimental capacity for Clstorage that is stored in 1 g of Bi using the Bi electrodes used in thisstudy is calculated to be 82.9 mg/g_(Bi) (=169.6 mg/g_(Bi)×48.9%).

Capacity for Cl Storage by CDI.

The capacity for Cl storage of CDI can be calculated from its capacityfor NaCl storage reported in the previous study, which is 13.5mg/g_(carbon). (J. Lee, S. Kim, C. Kim, J. Yoon, Hybrid capacitivedeionization to enhance the desalination performance of capacitivetechniques, Energy Environ. Sci. 7, 3683-3689 (2014).) Assuming that theactivated carbon electrodes used for the Na capture and the Cl capturehave the same mass and surface area, the capacity of storing 13.5 mg ofNaCl in 1 g of carbon is equivalent to storing 8.18 mg of Cl in 0.5 g ofcarbon and 5.32 mg of Na in 0.5 g of carbon. Therefore, the capacity forCl storage can be calculated to be 16.36 mg/g_(carbon).

The as-deposited Bi electrode contains a trace amount of Cl, suggestingthat it contains a trace amount of BiOCl, although BiOCl peaks were notdetected by the XRD of the as-deposited Bi electrode. Deposition ofBiOCl during deposition of Bi is possible because the plating solutionused for Bi deposition contained Cl⁻ ions as well as Bi³⁺ ions. Theconcurrent H₂ evolution during Bi deposition can increase the local pHat the WE surface, decreasing the solubility of Bi³⁺ and triggering theprecipitation of BiOCl.

After the 1^(st) chlorination, the Bi:Cl ratio is ˜1:1 confirming theformation of BiOCl. A slightly higher Bi content than 1:1 is due to thepresence of electrochemically inactive Bi present underneath BiOCl.After the 1^(st) dechlorination, the content of Cl⁻ became negligible,confirming the full conversion of BiOCl to Bi. The same behavior wasobserved for the 2^(nd) cycle.

The word “illustrative” is used herein to mean serving as an example,instance, or illustration. Any aspect or design described herein as“illustrative” is not necessarily to be construed as preferred oradvantageous over other aspects or designs. Further, for the purposes ofthis disclosure and unless otherwise specified, “a” or “an” means “oneor more”.

The foregoing description of illustrative embodiments of the inventionhas been presented for purposes of illustration and of description. Itis not intended to be exhaustive or to limit the invention to theprecise form disclosed, and modifications and variations are possible inlight of the above teachings or may be acquired from practice of theinvention. The embodiments were chosen and described in order to explainthe principles of the invention and as practical applications of theinvention to enable one skilled in the art to utilize the invention invarious embodiments and with various modifications as suited to theparticular use contemplated. It is intended that the scope of theinvention be defined by the claims appended hereto and theirequivalents.

What is claimed is:
 1. A method for removing chloride ions from anelectrolyte solution using an electrochemical cell comprising: achloride-storage electrode comprising bismuth metal; an electrolytesolution comprising chloride ions in contact with the chloride-storageelectrode; a second electrode in electrical communication with thechloride-storage electrode; and an electrolyte solution in contact withthe second electrode, wherein the electrolyte solution in contact withthe second electrode is either the electrolyte solution comprisingchloride ions that is in contact with the chloride-storage electrode ora different electrolyte solution, the method comprising: generating aflow of electrons from the chloride-storage electrode to the secondelectrode, whereby bismuth in the chloride-storage electrode is oxidizedto form BiOCl and a reduction reaction occurs at the second electrode.2. The method of claim 1, wherein the electrolyte solution comprisingchloride ions comprises salinated water.
 3. The method of claim 2,wherein the electrolyte solution comprises seawater.
 4. The method ofclaim 2, wherein the second electrode is a sodium-storage electrode andthe reduction reaction comprises reducing the sodium-storage electrodeto store sodium ions.
 5. The method of claim 2, wherein the secondelectrode is a hydrogen evolution electrode and the reduction reactioncomprises reducing water to hydrogen.
 6. The method of claim 2, whereinthe second electrode is an oxygen reduction electrode and the reductionreaction comprises reducing oxygen to hydroxide ions or hydrogenperoxide.
 7. The method of claim 1, further comprising: generating areverse flow of electrons, such that electrons travel from the secondelectrode to the chloride-storage electrode, whereby the BiOCl isreduced to Bi and chloride ions are released from the chloride-storageelectrode and an oxidation reaction occurs at the second electrode. 8.The method of claim 7, wherein the second electrode is a sodium-storageelectrode and the oxidation reaction comprises oxidizing thesodium-storage electrode to release sodium ions.
 9. The method of claim7, wherein the oxidation reaction is the oxidation of a metal with anoxidation potential.
 10. The method of claim 7, wherein the oxidationreaction comprises oxidizing chloride to chlorine gas.
 11. The method ofclaim 7, wherein the oxidation reaction comprises oxidizing an organicmolecule.